The present invention relates to voltage controlled oscillator (VCO) modulation. More particularly, this invention relates to varactor-based VCO fm-modulation
Voltage controlled oscillator (VCO) circuits are well known in the art and are utilized in a number of applications. For example, VCO circuits are used in phase-locked loop (PLL) circuits in high frequency applications such as wireless communications. A PLL is a component used in communications circuitry that enables communications equipment to quickly xe2x80x9clockxe2x80x9d onto a specifically selected frequency, typically the carrier frequency over which communications are sent. This fast locking ability is particularly important for devices such as cellular telephones, where the cell phones are desired to instantly switch carrier frequencies when traveling through different cellular zones or xe2x80x9ccellsxe2x80x9d. A VCO is an essential component of a PLL, whose output voltage is controllable by the application of an input control voltage. However, a VCO is very sensitive to fluctuations in a control voltage. The sensitivity of a VCO is typically expressed as MHz per volt.
Typically, a VCO circuit includes a variable element such as a capacitor that may be varied to adjust the frequency of an output signal of the VCO circuit. In a LC tank based VCO circuit, the frequency of the VCO circuit is determined by the inductance (L) and capacitance (C) of the tank circuit. By utilizing a varactor to function as a capacitor in the LC tank circuit, the capacitance of the VCO circuit can be varied by changing the voltage applied to the varactor. Thus, the frequency of the LC tank based VCO circuit is varied accordingly. However, the transfer characteristic of varactors is similar to reverse biased diodes which have a nonlinear transfer characteristic. Thus, a symmetric voltage swing would result in an asymmetric deviation. Accordingly, sensitivity of a VCO is affected due to a non-linearized varactor.
FIG. 1 shows a schematic diagram illustrating a conventional LC tank based VCO circuit. VCO circuit 20 generally includes an active device, such as an amplifier 22, and a LC tank circuit 30 (labeled in dash lines). The amplifier 22 includes a first input 24 adapted for receiving a first input signal and a second input 26 adapted for receiving a second input signal. In this case, the second input 26 is connected to ground. The output of the amplifier 22 is an input of the LC tank circuit 30. An output of the LC tank circuit 30 is fed back to the first input 24. The LC tank circuit 30 is formed by an inductor 32 having inductance Lo, a capacitor 34 having capacitance CLARGE and a variable capacitor or varactor 36 having capacitance C(V) which is controlled by a voltage V. The inductor 32 is connected between node 38 and ground. One end of the capacitor 34 is connected to node 38, and the other end of the capacitor 34 is connected to the varactor 36. The other end of the varactor 36 is connected to ground.
The LC tank circuit 30 is operable to provide an output signal 40, whose frequency is determined by the inductance L0, the capacitance CLARGE, and the capacitance C(V). The frequency is changed by varying the capacitance C(V) of the varactor 36 which is controlled by the voltage V. As stated above, because the varactor 36 inherently has a nonlinear transfer characteristic, a symmetric voltage swing would result in an asymmetric deviation of the frequency. As shown in FIG. 2, during modulation, the symmetric deviation in voltage, i.e., +xcex94V, xe2x88x92xcex94V, results in different deviation in capacitance, xcex94C1 and xcex94C2 (xcex94C1xe2x89xa0xcex94C2), respectively, which in turn results in different frequency deviation. The resulting frequency f(V) equals to [L0xc3x97Cxe2x80x2(V)]xe2x88x92xc2xd, wherein L0 is the inductance of the inductor 32, and Cxe2x80x2(V) is the equivalent load capacitance of the capacitor 34 and the varactor 36 in series.
In order to solve the asymmetry problem, there is a need to provide an improved VCO circuit for equalizing the positive frequency deviation and the negative frequency deviation for a varactor-based VCO fm-modulation.
It is with respect to this and other considerations that the present invention has been made.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention provides a technique for equalizing a positive frequency deviation and a negative frequency deviation for a varactor-based VCO fm-modulation. In one embodiment, two varactors are used.
In one embodiment of the present invention, the VCO comprises an active device, such as an amplifier, having a first input and a second input, and a LC tank circuit coupled to an output of the amplifier. An output of the LC tank circuit is fed back to the first input of the amplifier, while the second input of the amplifier is connected to ground. In the LC tank circuit, first and second varactors are provided. The first varactor is in series connection with a first capacitor having a first fixed capacitance. The second varactor is in series connection with a second capacitor having a second fixed capacitance. The LC tank circuit further includes an inductor which is in parallel to the series of the first varactor and the first capacitor, and in parallel to the series of the second varactor and the second capacitor.
In operation in accordance with the present invention, the VCO may be operated in a normal condition and positive/negative deviation conditions. In the normal condition, the control voltage for a first varactor is set to VA such that a load capacitance in a first series is CA, while the control voltage for the other varactor is set to VB such that a load capacitance in a second series is CB. Thus, the total load capacitance in the normal condition is CA+CB. During the positive deviation, the control voltage for both varactors is set to VA such that the total load capacitance is 2CA. During the negative deviation, the control voltage for both varactors is set to VB such that the total load capacitance is 2CB. Accordingly, the deviation in the load capacitance, xcex94C, for both negative and positive deviation is equal to CAxe2x88x92CB. Thus, xcex94f is the same for both positive deviation and negative deviation. The nonlinearity of varactor based fm-modulation has thus been solved by providing a linearization scheme, such as using two varactors.